Insights into the P-to-Q conversion in the catalytic cycle of methane monooxygenase from a synthetic model system

Abstract

For the catalytic cycle of soluble methane monooxygenase (sMMO), it has been proposed that cleavage of the O–O bond in the (μ-peroxo)diiron(III) intermediate P gives rise to the diiron(IV) intermediate Q with an Fe 2 (μ–O) 2 diamond core, which oxidizes methane to methanol. As a model for this conversion, (μ–oxo) diiron(III) complex 1 ([Fe III 2 (μ–O)(μ–O 2 H 3 )(L) 2 ] 3+ , L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) has been treated consecutively with one eq of H 2 O 2 and one eq of HClO 4 to form 3 ([Fe IV 2 (μ–O) 2 (L) 2 ] 4+ ). In the course of this reaction a new species, 2, can be observed before the protonation step; 2 gives rise to a cationic peak cluster by ESI-MS at m / z 1,399, corresponding to the {[Fe 2 O 3 L 2 H](OTf) 2 } + ion in which 1 oxygen atom derives from 1 and the other two originate from H 2 O 2 . Mössbauer studies of 2 reveal the presence of two distinct, exchange coupled iron(IV) centers, and EXAFS fits indicate a short Fe–O bond at 1.66 Å and an Fe–Fe distance of 3.32 Å. Taken together, the spectroscopic data point to an HO-Fe IV -O-Fe IV = O core for 2. Protonation of 2 results in the loss of H 2 O and the formation of 3. Isotope labeling experiments show that the [Fe IV 2 (μ–O) 2 ] core of 3 can incorporate both oxygen atoms from H 2 O 2 . The reactions described here serve as the only biomimetic precedent for the conversion of intermediates P to Q in the sMMO reaction cycle and shed light on how a peroxodiiron(III) unit can transform into an [Fe IV 2 (μ–O) 2 ] core.